Evaluating long-term thermal performance and soil recovery in energy piles under various operational modes

Energy piles present a feasible solution for heating and cooling decarbonisation and offer economic advantages. However, their widespread adoption is constrained by limited understanding of long-term performance. This study employs a validated, fully coupled three-dimensional thermo-hydro-mechanical numerical model to investigate long-term thermal performance of energy piles under different operational strategies. The model examines continuous operation (Mode 1), seasonal stoppage (Mode 2), and an idealised daily stoppage mode (Mode 3) over both short-term (1-year) and long-term (10-year) cycles. Results show that daily stoppage (Mode 3) limits cumulative soil temperature drift to ±2 °C after ten years, compared to over 6 °C under continuous loading. Mode 3 also yields the most stable outlet fluid temperatures, outperforming Mode 1 by up to 2 °C in cooling efficiency. Thermal impacts are spatially confined, with 90 % of recovery occurring within a 4 m radius from the pile. Mode 3, as an ideal operational strategy, establishes a performance benchmark for ground recovery and system energy efficiency. These findings highlight the importance of load scheduling in mitigating ground thermal degradation and enhancing long-term performance. The results provide a basis for integrating energy piles into seasonal thermal storage or hybrid heating systems and guiding future smart control strategies.